1. If the apex of the triangle is not over the base (one of the base angles is obtuse), then extend the base line so that it goes beyond the point where the altitude would intersect.

2. Set a compass to a distance slightly larger than the altitude. Keep it at this setting for the next step of the construction.

3. With the point of the compass on the triangle's apex, mark the base line (and/or its extension) at the two points whose distance from the apex matches the compass/divider setting.

4. Set the compass to a distance larger than the distance between the points just marked. (Its former setting will do; you don't have to change it.) Keep this setting throughout the next step.

5. With the point of the compass on one of the marked points on the baseline, draw an arc below the baseline across the space between the marked baseline points. Do the same with the point of the compass on the other marked baseline point.

6. Using a straightedge, line up the apex of the triangle and the intersection point of the arcs just drawn. The altitude of the triangle is the line segment along the straightedge that extends from the apex to the baseline.

2. Set a compass to a distance slightly larger than the altitude. Keep it at this setting for the next step of the construction.

3. With the point of the compass on the triangle's apex, mark the base line (and/or its extension) at the two points whose distance from the apex matches the compass/divider setting.

4. Set the compass to a distance larger than the distance between the points just marked. (Its former setting will do; you don't have to change it.) Keep this setting throughout the next step.

5. With the point of the compass on one of the marked points on the baseline, draw an arc below the baseline across the space between the marked baseline points. Do the same with the point of the compass on the other marked baseline point.

6. Using a straightedge, line up the apex of the triangle and the intersection point of the arcs just drawn. The altitude of the triangle is the line segment along the straightedge that extends from the apex to the baseline.